These meters do not really measure the true tension of the string. They attempt to infer the tension from the force it takes to bend the string a given distance. They usually are anywhere from right on to as much as 10 lbs. below the reference tension. They are best used to measure tension loss over time by cking the string on a regular basis. I use mine to ck the tension on a frame that someone brings in to be restrung to see if it really has lost a lot of tension. They can also be used to ck consistency on the same racquet and string from string job to string job. I have cked many racquets with mine and have never broken a string.

Gaines gave a full answer. Here is kinda my funny version.
It works like those device that you pinch your tummy in order
to get your body fat %. It's not terribly accurate but quite accurate
within some statistical bound. They figured average body fat
percentage of thousands of people whose tummy was pinchable
with certain amount of force....

So the stringmeter measures the stiffness of your tummy fat,..
oops, I mean, the strung string and turns it into the average
tensions for wide variety of strings...
So I guess it's pretty good measure for relative tension loss
over time for a given string...

No, you won't pop the string with it unless you have defactive
string. Tennis string is stronger than you think and the "sawing"
effect from the cross string is the major reason why string pops....

I would recommend it if you experiment with strings and tensions a lot...